SIP WG J. Peterson
Internet-Draft NeuStar
Expires: April 26, 2003 October 26, 2002
S/MIME AES Requirement for SIP
draft-peterson-sip-smime-aes-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
RFC3261 currently specifies 3DES as the required minimum ciphersuite
for implementations of S/MIME in SIP. This document updates the
normative guidance of RFC3261 to require the Advanced Encryption
Standard (AES) for S/MIME.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. S/MIME Ciphersuite Requirements for SIP . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . . 8
Informative References . . . . . . . . . . . . . . . . . . . . 9
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The Session Initiation Protocol (SIP) specification (RFC3261 [1])
currently details optional support (a normative MAY) for the use of
secure MIME, or S/MIME (RFC2633 [4]). Since RFC3261 was published,
the S/MIME specification and the underlying Cryptographic Message
Syntax (CMS, RFC3369 [3]) have undergone some revision. Ongoing work
has identified AES as a ciphersuite that might be used for content
encryption in S/MIME.
The Advanced Encryption Standard (AES [7]) is widely believed to be
faster and more secure than Triple-DES (3DES), which has previously
been mandated for usage with S/MIME. AES is also believed to have
comparatively low memory requirements, which make it suitable for use
in mobile or embedded devices, an important use-case for SIP.
As an additional consideration, the SIP specification has a
recommendation (normative SHOULD) for support of Transport Layer
Security (TLS, RFC2246 [8]). TLS support in SIP requires the usage
of AES. That means that currently, implementations that support both
TLS and S/MIME must support both 3DES and AES. A similar duplication
of effort exists with DSS in S/MIME as a digital signature algorithm
(the mandatory TLS ciphersuite used by SIP requires RSA). Unifying
the ciphersuite and signature algorithm requirements for TLS and S/
MIME would simplify security implementations.
It is therefore desirable to bring the S/MIME requirement for SIP
into parity with ongoing work on the S/MIME standard, as well as to
unify the algorithm requirements for TLS and S/MIME. To date, S/MIME
has not yet seen widespread deployment in SIP user agents, and
therefore the minimum ciphersuite for S/MIME could be updated without
obsoleting any substantial deployments of S/MIME for SIP (in fact,
these changes will probably make support for S/MIME easier). This
document therefore updates the normative requirements for S/MIME in
RFC3261.
Note that work on these revisions in the S/MIME working group is
still in progress. This document will continue to track that work as
it evolves. By initiating this process in the SIP WG now, we provide
an early opportunity for input into the proposed changes and give
implementers some warning that the S/MIME requirements for SIP are
potentially changing.
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2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC2119 [2] and indicate requirement levels for
compliant SIP implementations.
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3. S/MIME Ciphersuite Requirements for SIP
The following updates the text of RFC3261 Section 23.3, specifically
the fifth bullet point. The text currently reads:
o S/MIME implementations MUST at a minimum support SHA1 as a
digital signature algorithm, and 3DES as an encryption
algorithm. All other signature and encryption algorithms MAY
be supported. Implementations can negotiate support for these
algorithms with the "SMIMECapabilities" attribute.
This text is updated with the following:
S/MIME implementations MUST at a minimum support RSA as a digital
signature algorithm, SHA1 as a digest algorithm, and AES as an
encryption algorithm (as specified in [5]. For key wrap, S/MIME
implementations MUST support the AES Key Wrap Algorithm ([6]). S/
MIME implementations of AES MUST support 128-bit AES keys, and SHOULD
support 192 and 256-bit keys. Note that the S/MIME specification [4]
mandates support for 3DES as an encryption algorithm, DH for key
encryption and DSS as a signature algorithm. In the SIP profile of
S/MIME, support for 3DES, DH and DSS is RECOMMENDED but not required.
All other signature and encryption algorithms MAY be supported.
Implementations can negotiate support for algorithms with the
"SMIMECapabilities" attribute.
Transfer encoding used in S/MIME for SIP MUST use DER (Distinguished
Encoding Rules), not the Basic ASN.1 Encoding Rules.
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4. Security Considerations
The migration of the S/MIME requirement from Triples-DES to AES is
not known to introduce any new security considerations.
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5. IANA Considerations
This document introduces no considerations for IANA.
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Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, May 2002.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[3] Housley, R., "Cryptographic Message Syntax", RFC 3369, August
2002.
[4] Ramsdell, B., "S/MIME Version 3 Message Specification", draft-
ietf-smime-rfc2633bis-01 (work in progress), June 2002.
[5] Schaad, J. and R. Housley, "Use of the AES Encryption Algorithm
and RSA-OAEP Key Transport in CMS", draft-ietf-smime-aes-alg-05
(work in progress), Jul 2002.
[6] Schaad, J. and R. Housley, "Advanced Encryption Standard (AES)
Key Wrap Algorithm", RFC 3394, Sept 2002.
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Informative References
[7] National Institute of Standards & Technology, "Advanced
Encryption Standard (AES).", FIPS 197, Nov 2001.
[8] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, Jan 1999.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
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Appendix A. Acknowledgments
Thanks to Rohan Mahy for review of this document.
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